Suggest Treatment For Burning Sensation At The Penile Tip

i am 23 yrs male. i have a slight burning sensation at the tip of my penis from past 3 months. it goes at some time and comes back in 2 or 3 hrs. it feels good for 10 to 15 mins after urinating. but again starts. it also feels good when i am masturbating and penis is erect. but again after ejacuation it starts burning. almost it burns for 15 hrs a day.i never had sex. but used to masturbate for 2 times daily for the past 8 years. and from 5 months i am observing foul smell in my urine. and the urine is foamy. my urine tests always show normal. blood sugar fasting is 98 mg/dl. the tip of the penis is never red or never noticed any blood in my urine. i drink 3 liters of water a day and ll urinate allmost around 5 to 6 times/day. many times the urine is clear. have consultd doctor twice but the medicines were of no use. is my masturbation caused this ? is it a serious problem ?